Low-stress molded gasket and method of making same

ABSTRACT

A molded discrete low stress gasket is constructed of restructured filled PTFE for use in corrosive or severe chemical environments under relatively low bolt loads. The gasket has a gasket surface and includes a raised outer sealing ring and a raised inner sealing ring. The gasket may constructed from a restructured filled PTFE material, with the sealing rings deforming at lower pressures than the remaining portions of the gasket.

FIELD

The present invention relates generally to the field of fluid sealingand gasketed joints and more particularly to low-stress molded gasketsfor use in fragile joints, such as plastic (e.g., polyvinyl chloride or“PVC”), fiber reinforced plastic (FRP) and glass lined pipe in which thebolt load is relatively low to prevent damage to the flange joint.

BACKGROUND

A gasket is a material or combination of materials clamped between twoseparable members or flanges of a mechanical joint. The gasket functionsto effect a seal between the flanges and maintain the seal for anextended period of time. FIG. 1 is a cross -sectional view of a gasket20 clamped between two flanges 24, 28 of a mechanical joint 32. Theflanges 24, 28, are secured together with bolt 36. FIG. 1 alsoillustrates common forces that may affect the joint 32, such as boltload X, hydrostatic end force Y, and blowout pressure Z. The gasket 20,in many applications, must be capable of sealing the mating surfaces 40,44, and be impervious and resistant to the sealed media. Such gaskets 20also must be able to withstand the application of elevated temperatureand pressure in many applications. PVC and FRP piping are commonly usedin corrosive applications, such as encountered in chemical plants. Itwill be appreciated that piping systems using these materials aresomewhat fragile and require a gasket that will effect a seal atrelatively low bolt loads so as not to crack or otherwise damage theflanges. The gasket must also be dimensionally stable so as to maintaina seal during a range of possible thermal changes in the process andhave broad chemical compatibility.

Prior attempts to address the problems associated with gaskets for usein fragile joints have included, for example, envelope gaskets, rubbergaskets, rubber/polytetraflouroethylene (PTFE) gaskets, filled PTFEsheet, microcellular/porous PTFE sheet, and composite PTFE sheet. PTFEis commonly employed for gasketing in severe or corrosive chemicalenvironments as it has a number of desirable properties for use as agasketing material. For example, PTFE is inherently tough, chemicallyinert, has good tensile strength, and is stable over a broad range oftemperatures. However, pure PTFE polymer is not highly compressibleunder low flange loads, and also is prone to creep, both of which mayresult in loss of sealing pressure. Envelope gaskets are a compositestructure consisting of a PTFE envelope which is filled with a morecompressible filler such as compressed fiber or felt. The PTFE envelopeprovides chemical resistance while deformability is provided by thefiller material. However, PTFE envelopes are relatively thin (0.010 to0.020 inch) and can develop pin holes during manufacture or while inservice, thereby exposing the filler to incompatible corrosive media,which may result in loss of sealing pressure. Such envelope gaskets alsohave the least compressible component, i.e., the PTFE envelope which isnot highly compressible or deformable under low flange loads, asoutermost gasket surface.

Rubber gaskets are used routinely in plastic and FRP flanges because oftheir compressibility and resiliency, and their ability to seal atrelatively low bolt loads. However, rubber gaskets have limited chemicaland temperature resistance, and the proper compound must be specifiedfor each application. Thus, multiple process streams that use the samepiping are likely to require a time-consuming and somewhat costly changeof gaskets. Rubber/PTFE gaskets incorporate a bonded PTFE envelope atthe inner dimension of a rubber gasket. The envelope enhances thechemical resistance while the rubber substrate provides compressibilityand deformability. Again however, the PTFE envelopes are thin (0.010 to0.020 inch) and can develop pin holes during manufacture or while inservice, thereby exposing the rubber substrate to incompatible corrosivemedia. Likewise, the PTFE envelope which is not highly compressible ordeformable under low flange loads, is outermost in a rubber/PTFE gasket.

Filled PTFE sheets with good compressibility can be achieved byincorporating microballoons into the PTFE sheet material. Although PTFEsheet material offers the flexibility to be trimmed and modified by anend user, filled PTFE sheet material typically requires relatively highbolt loads to seal. Microcellular/porous PTFE sheets can be producedusing a number of techniques, one of which involves adding a filler tothe PTFE prior to trimming the sheet and then removing the filler afterthe sheet is formed. Thus, voids remain in the sheet material which giveit a desired porosity (i.e., microcellular PTFE). Another methodinvolves a particular sequence of extruding, stretching, and thenheating to form a product known as porous PTFE. However, microcellularand porous PTFE are generally very soft and flexible and can bedifficult to install in situations where limited flange separation ispossible. Further, because microcellular and porous PTFE sheets areporous, a gasket cut from either must be fully compressed to close offthe voids to prevent leakage through the gasket, and gaskets cut fromthese sheets typically require relatively high bolt loads to seal. Inorder to address the rigidity issues associated withmicrocellular/porous PTFE material, it has been proposed to laminatelayers of the porous microcellular and/or porous PTFE sheets to a metalor full density PTFE substrate, but testing has shown that thesematerials likewise require relatively high bolt loads to seal.

SUMMARY

Various aspects of the present disclosure provide a molded discrete lowstress gasket constructed of restructured filled PTFE for use incorrosive or severe chemical environments. Embodiments of the inventioninvolve cutting and heating a ring blank of the restructured filled PTFEto its gel point in a hatch oven, and thereafter transferring the blankto a compression mold which is at room temperature and which isconfigured to mold, for example, two or three raised concentric sealingrings into the opposing faces of the blank. The mold is quickly closedand the article is then cooled under a pressure of approximately 2000 to3000 pounds per square inch in a hydraulic press. The mold is thereafteropened and the article removed and trimmed using a steel rule die orequivalent cutting device. In the resulting gasket, areas adjacent therings are compressed to a greater extent during the molding process,resulting in higher densification of the filled PTFE. These regionsimpart strength and rigidity to the gasket, while the regions of thesealing rings are left substantially intact, yielding a region of highcompressibility.

Additional advantages and novel features of the invention will be setforth in part in the description which follows, and in part will becomeapparent to those skilled in the art upon examination of the following.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a gasket clamped between two flangesof a mechanical joint which illustrates forces affecting the joint, suchas bolt load, hydrostatic end force, and blowout pressure;

FIG. 2 is a perspective view of a low-stress molded gasket having twoconcentric sealing rings for an exemplary embodiment;

FIG. 3 is a top plan view of a low-stress molded gasket having twoconcentric sealing rings for an exemplary embodiment;

FIG. 4 is a cross-sectional view along section line A-A of FIG. 3;

FIG. 5 is an enlarged view of section B of FIG. 4;

FIG. 6 illustrates examples of comparative test results including agasket of an embodiment of the present disclosure and various othertypes of gasket materials;

FIG. 7 illustrates examples of additional comparative test results ofgaskets according to embodiments of the present disclosure over variousother types of gasket materials; and

FIG. 8 illustrates examples of further comparative test results ofgaskets according to embodiments of the present disclosure over variousother types of gasket materials.

DETAILED DESCRIPTION

The inventors of the present invention have recognized severalshortcomings of prior gasket materials, and have developed a gasketdescribed herein that has several features and advantages. For example,it is a feature and advantage of the present disclosure to provide amolded discrete low stress gasket constructed of restructured filledPTFE for use in corrosive or severe chemical environments. Anotherexemplary feature and advantage of the present disclosure to provide amolded low stress gasket for use in corrosive or severe chemicalenvironments that effects a seal at low loads thereby reducing possibledamage to fragile flanges that may result from relative high bolt loadsthat are required to effect a seal on traditional gaskets. A furtherexemplary feature and advantage of the present disclosure is to providea molded low stress gasket for use in corrosive or severe chemicalenvironments that has good recovery and maintains a seal when thermalcycled. An additional exemplary feature and advantage of the presentdisclosure is to provide a molded low-stress gasket that is highlycompressible and deformable to allow it to conform to distortions andundulations of mating flanges. Yet another exemplary feature andadvantage of the present disclosure is to provide a molded low-stressgasket that is rigid to facilitate installation. A still furtherexemplary feature and advantage of the present disclosure is to providea molded low-stress gasket that does not adhere to the joint flangesthereby simplifying replacement. Another exemplary feature and advantageof the present disclosure is to provide a molded low-stress gasket thathas a broad range of chemical and temperature resistance.

Reference is now made in detail to various exemplary embodiments of theinvention, one or more examples of which are illustrated in theaccompanying drawings. Each example is provided by way of explanation,and is not to be considered as a limitation of the invention. It will beapparent to those skilled in the art that various modifications andvariations can be made in various of the disclosed embodiments withoutdeparting from the scope or spirit of the invention. For example,features illustrated or described as part of one embodiment can be usedon another embodiment to yield a still further embodiment. Thus, it isintended that the present disclosure cover such modifications andvariations that come within the scope of the invention.

Embodiments of the present disclosure provide a molded PTFE gasket inwhich the base material is PTFE that is enhanced both by processing andthrough the incorporation of fillers to create a material that iscompressible and less prone to creep than pure PTFE. Further, variousexemplary embodiments involve incorporation of raised concentric sealingrings which are molded into the two opposing faces of the gasket. Suchraised concentric sealing rings act to reduce the contact area of thegasket and result in increased sealing stress under the sealing rings atreduced bolt loads.

A gasket according to embodiments of the invention may have two to threeconcentric sealing rings molded into its outer surfaces and having araised surface relative to other surfaces of the gasket, which reducethe contact area of the gasket on the flange. It is noted that otherembodiments may likewise have only one sealing ring or three or moresealing rings. The resulting flange load created by tightening theflange bolts is thus distributed over a relatively small surface arearesulting in relatively high stress in the regions of the sealing rings.The sealing rings thus enable the gasket to effect a seal at reducedbolt loading.

With reference now to FIGS. 2-5, an embodiment is described in which alow-stress molded gasket has two concentric sealing rings. FIG. 2illustrates a perspective view, and FIG. 3 illustrates a top plan view,of the gasket 100, that has a gasket surface 104 and includes an outersealing ring 108, and an inner sealing ring 112. The outer sealing ring108 and inner sealing ring 112 each have a surface that is raisedrelative to the gasket surface 104, and are also formed of material thatis more easily deformed than the material associated with the gasketsurface 104. The portions of the gasket associated with the gasketsurface 104 are referred to as ribs. The gasket 100 of this embodiment,and various other embodiments, is constructed from a restructured filledPTFE material that is commercially available from Garlock SealingTechnologies, Palymra, N.Y., under the trademark GYLON®. Therestructured filled PTFE of this embodiment includes polymeric chainshave a biaxial orientation, to provide improved mechanical propertiesand reduced creep, as compared to conventional PTFE compounds. Thefiller in one embodiment is a microballoon which enhancescompressibility to provide a tight seal when subjected to minimalpressure, such as reduced bolt loads in a fragile joint. The PTFE matrixwhich encapsulates the filler provides the chemically inert structure ofthe gasket 100. In such a manner, the gasket 100 has regions ofrelatively deformable material in sealing rings 108, 112, and has ribregions of relatively rigid material 104.

FIG. 4 illustrates a cross-sectional view along section lines A-A ofFIG. 3, and FIG. 5 is a magnified view of section B of FIG. 4. In thisexample, the gasket 100 has a circular configuration, with sealing rings108, 112, forming concentric rings within the gasket 100. The gasket 100has an outer diameter OD, and an inner diameter ID. Sealing ring 108 hasa ring diameter RD₁ and sealing ring 112 has a ring diameter RD₂. Thesealing rings 108 and 112, in this embodiment, are raised and have aheight h relative to surface 104, with the gasket having a thickness t₁relative to areas 104, and a thickness t₂ relative to the areas of thesealing rings 108 and 112. While a circular gasket 100 is illustrated,it will be readily understood that a gasket may be made in accordancewith the present disclosure in any of numerous different configurations.

A gasket is formed, in various embodiments, according to severalprocessing steps. A gasket of one embodiment is formed from a sheet ofrestructured filled PTFE of proper thickness, that is formed from knownprocessing techniques. The sheet of restructured filled PTFE is driedfor six hours at 225° F. to remove solvent that may be remaining in theformed sheet. A ring blank of inner and outer dimensions (ID and OD) iscut using a steel rule die, or equivalent cutting device. The blank isheated to its gel point of approximately 700° F. for fifteen minutes ina ventilated batch oven. Thereafter, the blank is rapidly transferredfrom the batch oven to a compression mold which is at room temperature.The compression mold is configured to mold raised concentric sealingrings 108 and 112 into the opposing faces of the gasket. The mold isquickly closed and the article is then cooled under a pressure ofapproximately 2000 to 3000 pounds per square inch in a hydraulic pressfor approximately one minute. The mold is thereafter opened and thegasket removed and finally trimmed using a steel rule die or equivalentcutting device.

The foregoing process, known as coining, together with the raisedconcentric sealing ring geometry creates regions of differingcompressibility/density within the gasket 100. Areas adjacent the rings108 and 112 are compressed to a greater extent during the coiningprocess resulting in higher densification of the filled PTFE. Theseregions impart strength and rigidity to the portions of gasket 100 thatcorrespond to gasket surface 104, also referred to as ribs. In theregions of the sealing rings 108 and 112, a reduced level ofdensification occurs to form the ring features, yielding regions of highcompressibility. It should he noted that the compressibility of the ribsmay be selected to support and transmit the bolt load to the opposingflange faces without excessive deformation, for various applications.

In alternate embodiments, other suitable fillers, such as bariumsulphate, silica, graphite, as well as microballoons, can be used toprovide desired mechanical properties and/or chemical resistance of thePTFE for various applications. Other embodiments may include metal orother material that is incorporated into the gasket. For example, tworings of perforated stainless steel sheet approx. 0.008″ thick may beencapsulated at an inner perimeter and an outer perimeter of the gasket.These rings can serve dual purposes of increasing the tensile strengthof the gasket to give it higher pressure capability and also acting as aheating element for in-mold sintering using an induction heating device.

FIG. 6 is a table illustrating examples of comparative test of gasketsaccording to embodiments of the present disclosure and various othertypes of gasket materials. The tests of FIG. 6 were performed based onthe DIN 3535 standard at low load and ambient temperature in whichleakage of gas through the seal under various gasket stresses andinternal pressures was collected and measured using a manometer.Referring further to FIG. 6, the gasket materials tested included themolded restructured microballoon filled PTFE gasket for variousembodiments of the present disclosure [Molded Gylon Gasket (Gylon3504)]; a finished flat sheet of restructured microballoon filled PTFE[Gylon 3504 (⅛″ Sheet)]; a molded restructured barium sulphate filledPTFE gasket for further alternate embodiments of the present disclosure[Molded Gylon Gasket (Gylon 3510)]; and a finished flat sheet ofrestructured barium sulphate filled PTFE gasket [Gylon 3510 (⅛″ Sheet)].

FIG. 7 is a table of additional comparative test results of gasketsaccording to embodiments of the present disclosure over various othertypes of gasket materials. Referring to FIG. 7, FRP flange testing wasperformed with a six inch 150 pound, full face FRP flange using water asa medium on gasket materials including a finished flat sheet ofrestructured microballoon filled PTFE [Gylon 3504 (⅛″ Sheet)]; afinished flat sheet with porous PTFE outer layers and full density PTFEinner layer [Gylon 3545 (⅛″ Sheet)]; a finished flat sheet with porousPTFE outer layers and full density PTFE inner layer with an unfilledPTFE envelope [Gylon 3545 w/PTFE Envelope]; an expanded PTFE gasket[Competitive Product Style 800]; and the molded restructuredmicroballoon filled PTFE gasket for embodiments of the presentdisclosure [Molded Gylon Gasket (Gylon 3504)].

FIG. 8 is a table of additional comparative test results of gasketsaccording to embodiments of the present disclosure over various othertypes of gasket materials. Referring to FIG. 8, thermocycling tests wereperformed on gasket materials including a finished flat sheet ofrestructured microballoon filled PTFE [Gylon 3504 (118″ Sheet)]; and themolded restructured microballoon filled PTFE gasket for embodiments ofthe present disclosure [Molded Gylon Gasket (Gylon 3504)].

It should be recognized that the various exemplary embodiments describedherein are merely illustrative of the principles of the presentinvention. Numerous modifications and adaptations thereof will bereadily apparent to those skilled in the art without departing from thespirit and scope of the present invention.

The invention claimed is:
 1. A method of making a gasket, the methodcomprising the steps of: providing a sheet of restructured filled PTFE;cutting a ring blank from the sheet of restructured filled PTFE; heatingthe ring blank to a gel point for the restructured filled PTFE;inserting the ring blank into a compression mold, which is at roomtemperature when the ring blank is inserted, and shaped to mold at leasttwo concentric, raised sealing rings into each of a pair of opposingfaces of the ring blank; the mold being further shaped to space theconcentric rings apart from one another, separated by compressedregions; cooling the ring blank within the compression mold underpressure, at room temperature, until the ring blank forms a gasketdefined by at least two concentric, raised sealing rings that are lessdense and more compressible than the compressed regions that separatethe rings; and removing the gasket from the compression mold.
 2. Themethod of claim 1 further comprising: drying the sheet of restructuredfilled PTFE to substantially remove any solvent that may be within thesheet of restructured filled PTFE.
 3. The method of claim 2 wherein thesheet of restructured filled PTFE is dried at an approximate temperatureof 225° F.
 4. The method of claim 1 wherein the ring blank is heated toa temperature of approximately 700° F.
 5. The method of claim 4 whereinthe ring blank is heated for approximately fifteen minutes.
 6. Themethod of claim 1 wherein the ring blank is cooled within thecompression mold under a pressure of approximately 2000 to 3000 poundsper square inch.
 7. The method of claim 6 wherein the ring blank iscooled within the compression mold under pressure for a period ofapproximately one minute.
 8. The method of claim 1 wherein therestructured filled PTFE of the sheet of restructured filled PTFE isfilled with microballoons.
 9. A method of making a gasket, the methodcomprising the steps of: cutting a ring blank from a sheet ofrestructured filled PTFE; heating the ring blank to a gel point for therestructured filled PTFE; inserting the ring blank into a compressionmold, which is at room temperature when the ring blank is inserted, andshaped to mold at least one raised sealing ring into each of a pair ofopposing faces of the ring blank; the mold being further shaped to formconcentric compressed regions on opposite sides of the at least oneraised sealing ring; cooling the ring blank within the compression mold,under pressure, at room temperature, for approximately one minute untilthe ring blank forms a gasket defined by at least two concentric,compressed regions that are more dense and less compressible than the atleast one raised sealing ring that separate the compressed regions.